1. Field of the Invention
The present invention relates to the 3rd generation mobile communication system. More particularly, the present invention relates to a communication system and method for turbo encoding and decoding of data
2. Description of the Related Art
Generally, next generation mobile communication systems based on code division multiple access (CDMA) mode support high speed data transmission of Mbps and require very low packet error rate of 10−5˜10−6 unlike voice communication systems. However, there are limits in applying the convolutional coding mode, generally used in existing communication systems, to high speed data transmission. In this respect, a turbo coding mode is widely used to maintain a low packet error rate even under a poor mobile communication environment. It is recent tendency that the turbo coding mode is widely adopted as the standard of a mobile communication system.
FIG. 1 shows a related art parallel concatenated turbo encoder for generating turbo codes. Referring to FIG. 1, a parallel concatenated turbo encoder consists of two identical constituent encoders 10 and 11 connected with each other in parallel and an interleaving unit 12 interposed between them. First parity bits Y0k and Y1k of a information bit Xk are generated from the first constituent encoder 10 while second parity bits Y0k′ and Y1k′ of an interleaved information bit Xk′ are generated from the second constituent encoder 11.
The related art parallel concatenated turbo encoder outputs information bits and parity bits in the order of Xk Y0k Y1k Xk′Y0k Y1k′ and consequently the coding rate of the turbo encoder is ⅙.
FIG. 2 shows an apparatus for generating a transmission signal based on a related art turbo encoder. Referring to FIG. 2, a basic turbo encoder 201 is provided with the parallel concatenated turbo encoder of FIG. 1. The basic turbo encoder 201 encodes the information bit sequence into an encoded bit sequence whose coding rate is ⅙. A puncturing unit 202 performs puncturing on the encoded bit sequence so that the resulting coding rate can be matched with the required coding rate. The puncturing is performed according to a predetermined puncturing pattern that gives the best performance. A rate matching unit 204 performs symbol repetition or puncturing on the punctured bits sequence to match the encoded bits sequence with the size N of the interleaving unit 205. An interleaving unit 205 interleaves the output bits sequence of the rate matching unit 204 to overcome the burst error characteristic of the channel. A modulator and spreading unit 206 modulates and spreads the interleaved bits sequence.
At this time, the encoded bits sequence from the basic turbo encoder 201 has an output order such as “Xk Y0k Y1k Xk′Y0k′Y1k′” punctured with the puncturing pattern determined by the required coding rate. As a result, the transmitted signal from the transmitter based on information bits sequence has one output order.
However, there are cases where differently encoded bit sequences are needed that are generated from the same information bits sequence and have same error correcting capabilities. For instance, if two antenna transmit diversity is applied to the related art, it is better for each antenna to transmit different signals than to transmit the identical signals since additional code combining gain as well as maximal ratio combining gain can be obtained at the receiver.
In the related art, since the basic turbo encoder 201 has the output order such as “Xk Y0k Y1k Xk′Y0k′Y1k′”, the puncturing unit 202 in the transmitter does not have one puncturing pattern but several puncturing patterns to obtain the encoded bit sequences differently. Further, the punctured bit sequences must have the same error correcting capabilities. It is very difficult to find the puncturing patterns that have the same performance and even if it is found, the transmitter becomes more complex because the change in the puncturing pattern used in the puncturing unit 202 can cause the puncturing pattern of the rate matching unit 204 to change. The transmitter needs to memorize the combinations of puncturing patterns that are used in both the puncturing unit 202 and the rate matching unit 204.
FIG. 3 shows a related art apparatus for decoding a received signals when identical signals are used for transmission diversity.
Referring to FIG. 3, the two modulated signals transmitted from the transmitter are received by two receiver antenna. Then, the signals are demodulated and despread by a corresponding demodulator and a corresponding despreading unit 301 and 302. The output signals from the demodulator and despreading units 301 and 302 are soft combined with each other by a combiner 303 to get a bit sequence with low error rate. The combined bit sequence is deinterleaved and rate dematched through a rate dematching unit 304 and depunctured through a depuncturing unit 305. The resultant bit sequence is decoded to get the unit 305 originally transmitted information bit sequence Xk.
A detailed example is shown in the following to illustrate the idea more definitely.
Two antenna transmit diversity is assumed and the desired coding rate after the puncturing unit 202 is assumed to be ½.
To obtain a desired coding rate from the ⅙ original code rate, the symbol puncturing block punctures the bits according to the puncturing pattern (not shown). In the case that identical signals are transmitted via two antennas, the encoded bit sequence from the turbo encoder 201 is punctured according to the puncturing pattern and the resultant sequence is in the order “Xk Y0k Xk+1 Y0k+1′”. The punctured sequences are rate matched, interleaved, modulated and transmitted via the two transmit antennas through the noisy channel.
The two signals go through different channel environments and are received at the receiver antenna separately. The combiner 303 combines the power of the demodulated bit sequences from two antenna in the order as they are transmitted, i.e. “Xk Y0k Xk+1 Y0k+1′”.
In other words, the combiner 303 performs maximal ratio combining on the demodulated bit sequences and the effective coding rate does not change compared with that of the transmitted signal.
In the case that different signals are transmitted via two antenna, the encoded bit sequence from the turbo encoder 201 is punctured by different puncturing patterns that give the same performance. Assuming that puncturing patterns “110000100010” and “100010110000” are found to have same performance, two different signals can be generated from them. Namely, the encoded bit sequence from the turbo encoder 201 is punctured using the different puncturing patterns. The resultant sequences are in the order “Xk Y0k Xk+1 Y0k+1′” and “Xk Y0k′Xk+1 Y0k+1”. These sequences are rate matched, interleaved, modulated and transmitted via two transmit antenna through the noisy channel. The two signals are received at the receiver, demodulated, despread, deinterleaved, depunctured separately and combined at the combiner. In this case, the combiner should be intelligent enough so that it can perform maximal ratio combining to the systematic bits while performing code combining to the parity bits, i.e. the combined bit sequence is in the order “Xk Y0k Y0k′Xk+1 Y0k+1′Y0k+1”. In other words, the combiner performs code combining as well as maximal ratio combining on the received signals and the effective coding rate reduces to 1/3, which enhances the error correcting capability.
Another example can be shown in the case that a hybrid type automatic repeat request (referred to as “H-ARQ”) mode is used.
FIG. 4 is the flow chart illustrating the procedure of a related art hybrid type ARQ mode.
Referring to FIG. 4, in the related art hybrid type ARQ mode, a transmitter adds a cyclic redundancy check (CRC) code to the information bit sequence, and the information bit sequence to which the CRC is added is encoded to generate a encoded bit sequence. The encoded bit sequence is then punctured, rate matched, interleaved, modulated into a transmission signal and transmitted to the receiver. Then, the receiver receives the signal (S20) and demodulates and decodes the signal. The receiver determines whether the demodulated bit sequence is “New signal” or “Retransmitted signal” by examining if the same bit sequence as the demodulated bit sequence of the receiver exists in a buffer (S21).
If the same bit sequence does not exist in the buffer, the received bit sequence is directly stored in the buffer (S22). If the same bit sequence exists in the buffer, the received bit sequence is combined with the bit sequence in the buffer and then stored in the buffer (S23).
The bit sequence stored in the buffer is checked if there is any error in the received bit sequence by CRC (S24). If no error exists in the received bit sequence, the receiver transmits an acknowledgement (ACK) signal to the transmitter and empties the buffer (S26).
If any error exists in the received bit sequence, the received bit sequence is stored in a buffer, and the receiver transmits a non-ACK(NACK) signal to the transmitter (S25).
Accordingly, if the transmitter has received the ACK signal, the transmitter transmits the next new information data. If the transmitter has received the NACK signal, the transmitter retransmits the previously transmitted information data.
The receiver demodulates the retransmitted signal and combines it with the bit sequence stored in the buffer, so that the received bit sequence is decoded. If no error exists in the combined bit sequence, the receiver transmits the ACK signal to the transmitter. If any error exists in the combined bit sequence, the receiver transmits the NACK signal to the transmitter to request another retransmission of the same data, and at the same time stores the combined bit sequence in the buffer.
At this time, if the retransmitted bit sequence includes the same parity bits as the bit sequence previously transmitted, the combining gain that can be obtained at the receiver is only maximal ratio combining gain in the AWGN (Additive White Gaussian Noise) channel. To get code combining gain as well as maximal ratio combining gain, “Retransmitted bit sequence” must include different parity bits from that of “New bit sequence” and the puncturing unit of the transmitter must have several patterns to generate sequences that include different parity bits.
Getting differently encoded sequences based on a information source is beneficial when the encoded sequences are used in the transmit diversity system. However, the method of getting differently encoded sequences by changing the puncturing patterns of the transmitter can add complexity because both the transmitter and the receiver should use not one puncturing pattern but several puncturing patterns. Besides, it is difficult to find puncturing patterns that have the same performance.